1. Technical Field
The present invention relates in general to magnetic hard disk drives and, in particular, to an improved system, method and apparatus with multiple anisotropy layered magnetic structures for controlling reversal mechanism and tightening of the magnetic switching field distribution in bit patterned media. Specific examples include three or more layers with different anisotropy origin, anisotropy direction or anisotropy gradients to further increase writeability gains in bit patterned media.
2. Description of the Related Art
Bit patterned media (BPM) is a leading candidate to extend the densities of magnetic recording beyond those achievable by conventional continuous perpendicular magnetic recording based on granular recording media. The islands of BPM need to be sufficiently small and of sufficient magnetic quality to support high bit areal densities (e.g., at 500 Gb/in2 and beyond). For example at a density of 1 Tb/in2, the islands have diameters that are approximately 15 to 20 nm (assuming a unit cell of 25.4 nm2) with trenches having widths of about 10.4 to 15.4 nm, and bit aspect ratio (BAR) of about 1 or more. Moreover, we expect that the switching field distribution (SFD) needs to be smaller than 1000-1500 Oe, depending on the head field gradient and other system parameters. See, e.g., M. E. Schabes, “Micromagnetic Simulations for Terabit/in2 Head/Media Systems,” J. Magn. Mag. Mat., (2008). Furthermore, since the field of the write head becomes smaller as the size of the write head is decreased, maintaining the writeability and thermal stability of the islands is an issue for BPM at densities of 1 Tb/in2 and beyond.
Another critical issue for the development of BPM is that the SFD (i.e., the bit-to-bit variation of the coercive field) needs to be narrow enough to secure exact addressability of individual predefined bits without overwriting adjacent bits. The SFD has many origins, such as variations in the patterned dot sizes, shapes and spacings, intrinsic magnetic anisotropy distribution of the magnetic thin film system used, and dipolar interactions between bits.
It also is known that exchange spring multilayer structures provide writeability gains for approximately fixed thermal barriers, and have thus been proposed for recording systems using continuous media and bit patterned media. See, e.g., D. Suess, et al., Appl. Phys. Lett. 87, 012504 (2005); D. Suess, Appl. Phys. Lett. 89, 113105 (2006); D. Suess, et al. J. Magn. Magn Mater. 290-291, 551 (2005); D. Suess, et al., Appl. Phys. Lett. 92, 173111 (2008). Multilayer exchange spring recording media comprising a hard magnetic storage layer strongly exchange coupled to a softer nucleation host have been proposed to decrease the switching field of the storage layer. This design keeps the energy barrier of the hard layer almost unchanged which allows maintaining a good thermal stability while lowering the reversal field. See, e.g., U.S. Pat. App. No. 2007/0292720, which is incorporated herein by reference in its entirety. In such a dual hard/soft layer structure, a vertical domain wall is nucleated at a low magnetic field in the softer layer. The vertical domain wall propagates through the soft layer and is pinned at the interface to the hard layer until the magnetic field amplitude is large enough to “propagate” the domain wall into the hard layer (i.e., the actual storage layer). In that case, the media switching field is defined as the domain wall depinning field. This field is lower than the media switching field itself, so that the exchange-spring structure allows decreasing of the media switching field. Moreover, the dependence of the depinning field as a function of the external field angle θ relative to the anisotropy axis is described by the Kondorsky-like law, Hswitching=1/cos(θ). In that case, Hswitching does not vary much as θ increases from 0 to 45 degrees, so that the exchange spring media allows reducing the SFD originating from an easy anisotropy axis angular distribution in the magnetic media.
In the present invention, novel structures of BPM islands are disclosed that enhance the gains of exchange spring materials for BPM, and thereby provide a solution for the aforementioned problems of BPM at areal densities in the range of about 0.5 to 10 Tb/in2.
The local applied field required for propagating the vertical domain wall depends mainly on the properties of the media layer at the interface to the soft layer within a depth equal to the exchange length: L(ex)=[A/(2πMs2)]1/2. This is about 20 nm for a Co/Pd multilayers, assuming an exchange constant A=4·10−6 erg/cm, and a saturation magnetization Ms=400 emu/Cm3. This means that any intrinsic or extrinsic defect within the exchange length L(ex) inside the media layer induces a change (mostly an increase) in the propagation field value. In a real patterned dot array, the media layer is rarely uniform in depth and has a large intrinsic anisotropy variation from one bit to another. See, e.g., T. Thomson, et al., Phys. Rev. Lett. 96, 257204 (2006). Exchange-spring media structures still present large switching field distributions due to the large media volume that controls the propagation fields.
To solve at least part of this problem, one aspect of the present invention (beyond the introduction of more general, different anisotropy multilayer structures) is to reduce the magnetic volume that controls the domain wall pinning by adding a thin (high anisotropy) layer between the soft nucleation host layer and the actual media layer, acting as a potential barrier for the domain wall propagation. In that case the media layer is no longer the layer defining the domain wall pinning features and the properties of the pinning layer and actual storage layer can be tuned and optimized independently.